The cooling tower of the Nyongbyon nuclear complex is demolished in Nyongbyon, also known as Yongbyon, North Korea, June 27, 2008. North Korea vowed today to restart a nuclear reactor that can make one bomb's worth of plutonium a year, escalating tensions already raised by near daily warlike threats against the United States and South Korea.

North Korea on Tuesday said it will restart a shuttered nuclear reactor capable of producing fissile material for weapons and “readjust” other facilities at its sprawling Yongbyon nuclear facility some 55 miles north of Pyongyang.

Is this just empty rhetoric from a regime trying to test the resolve of South Korean and US leaders? Or is it a genuinely worrisome development in an increasingly tense area of the world?

The answer to that question may depend upon North Korea’s follow-through. It’s possible the warning is just words and that construction at Yongbyon won’t actually accelerate. It’s also possible the Pyongyang regime will now rebuild the facility that produced the plutonium for at least two of North Korea’s nuclear tests. That would not be a good thing.

The main issue may be what happens to the Yongbyon plutonium production reactor. This facility, which first went critical in 1985, can produce enough dangerous fissile stuff for about one small nuclear weapon per year. It was shut down in 2007 per international disarmament talks (which have since stalled). In 2008 North Korean officials made a show of imploding its aging cooling tower, issuing rare invitations to international journalists to visit and see the destruction.

The reactor has not been ripped up beyond hope of repair, though. Far from it. Restarting this facility would take about six months, according to Sig Hecker, a nuclear expert from Stanford’s Center for International Security and Cooperation who has visited the site.

Mr. Hecker estimates that North Korea has about four to eight bombs worth of plutonium extracted from fuel rods used to power this reactor in the past. A restart obviously would allow Pyongyang to slowly build up this arsenal.

The second issue of “readjustment” might allow this increase to occur more quickly. That’s because this statement suggests that North Korea will do more to produce highly-enriched uranium, the second type of fissile material used in nuclear weapons.

There’s already a centrifuge facility for uranium enrichment operating at Yongbyon. North Korean officials surprised Hecker by taking him on a tour of the previously-undetected plant in 2010. At the time the officials said it was meant to produce fuel for a light-water reactor also under construction at the site. The “readjustment” term implies that Pyongyang may up the ante and openly begin production at bomb-worthy enrichment levels.

The US has long suspected that North Korea was enriching uranium in secret. Centrifuge halls are easy to hide and North Korea’s mountains have many tunnels where they could be stashed. In an address to a South Korean nuclear conference, Hecker expressed doubt that the Yongbyon enrichment facility sprang up from nowhere within months.

“The Yongbyon centrifuge facility could not have been constructed from scratch and made operational in only 18 months, between April 2009 and November 2010, as Pyongyang has claimed,” Hecker said at the conference. “It is likely that the North had one full cascade (about 340 centrifuges) operational at a separate site long before it moved into the renovated Yongbyon fuel fabrication building and revealed their centrifuge program in November 2010.”

Tuning up the Yongbyon centrifuges could produce enough highly-enriched uranium for one bomb per year. Any clandestine facility likely could produce at least that much, if not more. Add in the plutonium production from a restarted reactor, and some US experts see a worrisome trend, if not a nightmare scenario: a North Korea eager to produce a large and workable arsenal of nuclear weapons.

It’s possible Pyongyang isn’t technically good enough to get both these fissile material tracks going. In the Washington Post, Max Fisher writes that the vow to restart the plutonium reactor in fact indicates that North Korea can’t master the difficult art of uranium enrichment.

And again, it’s possible the whole thing is just talk. But if it isn’t, the US and its allies may find it hard to stop the move. Current sanctions are leaky at best due to China’s “inattention,” writes Mr. Fitzpatrick, the IISS nuclear expert. And in any case North Korea is by now almost self-sufficient in nuclear technology.

“Unfortunately, there are not good solutions,” writes Fitzpatrick. “The only glimmer of hope is that North Korea also announced that along with the nuclear weapons work it will simultaneously be pushing forward economic development.”